Passive Optical Networks or PONs are more and more replacing traditional physical layer solutions such as twisted pairs or telephone lines and coaxial cable. Optical fibres offer superior bandwidth performance compared with these traditional solutions. The phased integration of PONs has started from the network provider up to the end user premises whereby optical fibre is used to connect the network provider's backbone to distribution points. From these distribution points onwards, traditional wired lines are then used to bridge the last mile towards the end user node. A typical example of such a distribution point is a Digital Subscriber Line Access Multiplexer or DSLAM connecting multiple Digital Subscriber Lines or DSLs to the network provider's backbone by a PON. In densely populated areas or where there are customers requiring high bandwidths, network operators are now even offering PONs up to the end user's premises.
Optical fibres are however prone to physical defects such as bends, a misalignment of contacts and loose, dirty or damaged connectors. Therefore, to ensure good Quality of Service or QoS, it is important to monitor the performance of the optical fibre allowing timely intervention of a technician.
One way of assessing impairments in a PON is by Optical Time Domain Reflectometry or OTDR which is a technique that allows identifying potential defects. However, to provide accurate results, it requires quite long monitoring periods resulting in a long downtime for the end user.
To alleviate the long monitoring periods and downtime periods, embedded OTDR or eOTDR was introduced. With eOTDR, the service does not need to be interrupted and is therefore referred to as a non-intrusive diagnosis technique. Moreover, non-intrusive techniques allow detecting transient phenomena, such as sudden degradation of the fibre, occurring during operation and thus not detectable through a separate monitoring cycle.
However, as both OTDR and eOTDR make use of the reflection characteristics of light, they have the inherent disadvantage that it is difficult to assess the location of the fault. For example, if a fault occurs behind a splitter in the PON, it is difficult to assess in which fibre after the splitter the fault is located.
Therefore, in order to face this issue, other ways of performing diagnosis have been explored. One group of solutions performs monitoring and diagnosis by data analysis of PON operational parameters typically provided by the Physical Layer or PHY in the transceivers connected at the ends of the optical fibre. As the PHY operational parameters are exchanged during operation, these diagnosis techniques are also non-intrusive. The main advantage of using PON operational parameters over eOTDR and OTDR is these operational parameters are available per transceiver and thus per optical link contained in the entire PON network. Therefore, this technique allows a more precise localization of the fault.
One such a solution is illustrated in FIG. 1 and is based on the optical received power or RxPower over time as denoted in the figure, preferably measured at both sides of the optical fibre. There, the appearance of a fault on the fibre is detected by a significant transient step in the RxPower, as for example the case for fault 1. Then, a fault is considered as resolved when the RxPower for the current optical link is restored above a predefined power threshold value, i.e. the Optical Link Healthy threshold. This solution thus allows assessing the global health status of the optical fibre. FIG. 1 shows three such thresholds by the horizontal dashed lines of which only the middle one is correctly chosen.
A disadvantage of the above solution as illustrated by FIG. 1 appears when there is a bias between the optical link healthy threshold and the fault effect. This could cause the RxPower to drop below the optical link healthy threshold even in a healthy state, i.e. when there are no faults at all on the fibre. Also the opposite can happen where the RxPower is above the optical link healthy threshold even when there is a fault occurring on the optical fibre. These two cases are illustrated by the upper and lower optical link healthy threshold in FIG. 1.
Another disadvantage of this solution is that it is not able to discriminate the faults and therefore does not propose an evaluation of the respective life-cycle of the faults. Instead, the global optical link health is evaluated without dedicated information about each fault states. Therefore, it does not allow detecting reparation of a particular fault, it does not allow proposing improvement steps to the field technician during his troubleshooting process nor guiding him and at the end thereby making him blind to any improvements up to the time where the global optical link health is restored.
Finally, the current existing solution cannot make the distinction between the different nature of faults and thus the type of faults occurring such as for example if a fibre is bent or by a dirty connector. Therefore information needed for or about the recovery of a fault is missing, leading at the end the field technician into mistakes.
In the publication of the patent application EP2579480A1, a method is disclosed to derive a type of fault occurring in an optical network by collecting measurement data based on the received signal. However, it does not disclose how to obtain the life-cycle of the fault nor how to differentiate between faults of the same type.
It is therefore an object of the invention to solve or alleviate the above disadvantages and to provide a way to track the life-cycle of individual faults on an optical fibre.